Correction for errors caused by variation in water conditions

ABSTRACT

Method for processing seismic data to correct for errors caused by variation in water conditions. In one implementation, the method may include (a) applying a dip correction to a plurality of observed water bottom reflection times using a model water velocity and an estimate of geologic dip; (b) applying a normal moveout (NMO) correction to the dip corrected observed water bottom reflection times using the model water velocity; (c) applying a common mid point (“CMP”) bin centering correction to the NMO corrected, dip corrected observed water bottom reflection times using the model water velocity and the estimate of geologic dip; (d) solving for Δs i , which is an estimate of the difference in slowness between the observed water bottom reflection times and the water bottom reflection times that would have been observed had the water velocity been the same as the model water velocity; (e) solving for an estimate of observed water velocity based on s obs,i =s m +Δs i , where s obs,i  is an estimate of observed slowness and s m  is defined as the model slowness; (f) layer replacing the observed water bottom reflection times using the estimate of the observed water velocity and the model water velocity; and (g) repeating steps (a) to (f) using the layer replaced observed water bottom reflection times until the changes in the estimate of observed water velocity approach zero.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit of U.S. provisional patent applicationSer. No. 60/911,779, filed Apr. 13, 2007, which is incorporated hereinby reference.

BACKGROUND

1. Field of the Invention

Implementations of various techniques described herein generally relateto marine seismic surveying, and more particularly, to correcting forerrors caused by variation in water conditions.

2. Description of the Related Art

The following descriptions and examples do not constitute an admissionas prior art by virtue of their inclusion within this section.

Seismic exploration may be used to locate and/or survey subterraneangeological formations for hydrocarbon deposits. Since many commerciallyvaluable hydrocarbon deposits are located beneath bodies of water,various types of marine seismic surveys have been developed. In atypical marine seismic survey, such as the exemplary survey illustratedin FIG. 1, an array 100 of marine seismic streamer cables 105 may betowed behind a survey vessel 10 over a survey area 115. The seismicstreamer cables 105 may be several thousand meters long and contain alarge number of sensors 125, such as hydrophones and associatedelectronic equipment, which may be distributed along the length of theeach seismic streamer cable 105. The survey vessel 110 may also tow oneor more seismic sources 120, such as airguns and the like.

As the array 100 is towed over the survey area 115, acoustic signals, or“shots,” produced by the seismic sources 120 may be directed downthrough the water into the earth beneath (not shown), where they may bereflected from the various subterranean geological formations. Thereflected signals may be received by the sensors 125 in the seismicstreamer cables 105, digitized and then transmitted to the survey vessel110. The digitized signals may be referred to as “traces” and may berecorded and at least partially processed at the survey vessel 110. Theultimate aim of this process is to build up a representation of thesubterranean geological formations beneath the array 100. Analysis ofthe representation may indicate probable locations of hydrocarbondeposits in the subterranean geological formations.

Since the area of the array 100 is typically much smaller than thesurvey area 115, a representation of the earth strata in the survey area115 may be formed by combining data collected along a plurality of saillines 130(1-n). For example, a single survey vessel 110 may tow a singlearray 100 along each of the sail lines 130(1-n). Alternatively, aplurality of survey vessels 110 may tow a plurality of arrays 100 alonga corresponding plurality of the sail lines 130(1-n). However,variations in the water conditions, e.g. water temperature, salinity,and the like, between the plurality of sail lines 130(1-n) may causevariations in the velocity of sound in water among the sail lines130(1-n). For example, the variations in seismic travel time can be onthe order of 10 or 20 milliseconds for traces having a small distancebetween the source and detector for surveys carried out in deeperwaters, e.g., greater than 200 m. The variations in the seismic wavetravel times may shift the temporal position of the various eventsrecorded in the seismic data, such as reflections and refractions of theseismic waves from the subterranean geological formations beneath thearray 100. Consequently, the variations in the travel times may make itdifficult to analyze the combined seismic data set and may reduce theaccuracy of the survey.

Moreover, the data for the sail lines 130(1-n) may be collected atdifferent times. For one example, a single pass along one of the saillines 130(1-n) may take several hours to complete. As a result, if asingle survey vessel 110 is used, data for the first sail line 130(1)may be recorded at an earlier time than data for the last sail line130(n). For another example, inclement weather and/or high seas mayforce a survey to be suspended before resuming hours or days later. Foryet another example, historical data from previous surveys performedmonths or years earlier may be combined with new data to extend thesurvey or to fill in deficiencies in coverage that may be introduced bycurrents, obstacles such as platforms, and the like. And for yet anotherexample, data from repeat surveys may be used to analyze and monitorchanges in productive oil and/or gas reservoirs.

Combining data from different times, and especially from differentsurveys, may exacerbate the aforementioned difficulties associated withvariations in the velocity of sound in the water layer. For example,seasonal variations of the water temperature, salinity, and the like,may cause pronounced variations in the velocity of sound in water. Foranother example, shifts in water currents may cause unpredictablevariations in the velocity of sound in water, particularly for surveyscarried out near the edge of strong water currents.

The seismic data may be corrected for the variations in the velocity ofsound in water by computing one or more so-called Δt values, which aretypically defined as a difference between an expected travel time,usually based on an assumed ideal water velocity, and a measured traveltime for one or more seismic signals. For example, the assumed idealwater velocity may be a constant velocity or one with very smoothspatial changes in velocity.

In one conventional method for determining the Δt values (described inWombell, R., 1996, “Water Velocity Variations In 3-D SeismicProcessing,” 66th Ann. Internat. Mtg: Society of ExplorationGeophysicists, Expanded Abstracts, 1666-1669), normal move-out stackingvelocities and zero offset water bottom reflection times are computedalong adjacent sail lines. The velocities are then converted to zerooffset travel time differences using the formula:Δt=T_(w)(ΔV_(w)/V_(w)), where Δt is the difference in two-way traveltime at zero offset due to the change in water velocity, T_(w) is thezero offset water bottom reflection time, V_(w) is the reference watervelocity chosen by the practitioner, and ΔV_(w) is the differencebetween V_(w) and the computed stacking velocity. The Δt values are thenapplied to normal move-out corrected seismic data. One problem with thismethod is that the velocity analysis must be extremely accurate. Anotherissue is the effect of water bottom structure on the velocity analysis.If the dip of the water bottom, i.e., the angle the water bottom makeswith a horizontal plane, changes between or along sail lines, thecalculated velocities are strongly affected and may reduce the accuracyof the Δt calculation.

SUMMARY

Described herein are implementations of various techniques for a methodfor processing seismic data to correct for errors caused by variation inwater conditions. In one implementation, the method may include (a)applying a dip correction to a plurality of observed water bottomreflection times using a model water velocity and an estimate ofgeologic dip; (b) applying a normal moveout (NMO) correction to the dipcorrected observed water bottom reflection times using the model watervelocity; (c) applying a common mid point (“CMP”) bin centeringcorrection to the NMO corrected, dip corrected observed water bottomreflection times using the model water velocity and the estimate ofgeologic dip; (d) solving for Δs_(i), which is an estimate of thedifference in slowness between the observed water bottom reflectiontimes and the water bottom reflection times that would have beenobserved had the water velocity been the same as the model watervelocity; (e) solving for an estimate of observed water velocity basedon s_(obs,i)=s_(m)+Δs_(i), where s_(obs,i) is an estimate of observedslowness and s_(m) is defined as the model slowness; (f) layer replacingthe observed water bottom reflection times using the estimate of theobserved water velocity and the model water velocity; and (g) repeatingsteps (a) to (f) using the layer replaced observed water bottomreflection times until the changes in the estimate of observed watervelocity approach zero.

The above referenced summary section is provided to introduce aselection of concepts in a simplified form that are further describedbelow in the detailed description section. The summary is not intendedto identify key features or essential features of the claimed subjectmatter, nor is it intended to be used to limit the scope of the claimedsubject matter. Furthermore, the claimed subject matter is not limitedto implementations that solve any or all disadvantages noted in any partof this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

Implementations of various technologies will hereafter be described withreference to the accompanying drawings. It should be understood,however, that the accompanying drawings illustrate only the variousimplementations described herein and are not meant to limit the scope ofvarious technologies described herein.

FIG. 1 illustrates a schematic diagram of a typical marine seismicsurvey.

FIG. 2 illustrates a schematic diagram of a marine seismic survey areain connection with implementations of various techniques describedherein.

FIGS. 3A and 3B illustrate a side view of a survey vessel and a portionof the seismic array at two different locations in connection withimplementations of various techniques described herein.

FIG. 4 illustrates a plurality of midpoint cells in the marine seismicsurvey area shown in FIG. 2 in connection with implementations ofvarious techniques described herein.

FIG. 5 illustrates a plurality of signal paths from a surface through acommon midpoint cell on a sea floor and back to the surface inconnection with implementations of various techniques described herein.

FIGS. 6A, 6B, and 6C illustrate a flow diagram of a method forcorrecting seismic data due to errors caused by variations in the waterconditions in accordance with implementations of various techniquesdescribed herein.

FIG. 7 illustrates a computing system, into which implementations ofvarious technologies described herein may be implemented.

FIGS. 8A and 8B illustrate a cross sectional view of a plurality ofsignal paths perpendicular to the direction of geologic strike inaccordance with implementations of various techniques described herein.

DETAILED DESCRIPTION

The discussion below is directed to certain specific implementations. Itis to be understood that the discussion below is only for the purpose ofenabling a person with ordinary skill in the art to make and use anysubject matter defined now or later by the patent “claims” found in anyissued patent herein.

One goal of implementations of various techniques described herein is tosimplify and improve the process of determining and applying correctionsfor water velocity variation to seismic data. Conventional methods areprone to errors due to oversimplification of the problem, dipping andcomplex water-bottom geometry, and irregular survey geometry. Grosserrors generally require manual corrections. Smaller errors that areallowed to remain in the data often compromise the strength andsharpness of the final processed image.

FIG. 2 illustrates a marine seismic survey area 200 in connection withimplementations of various techniques described herein. To survey themarine seismic survey area 200, one or more survey vessels 210(1-2) maytow one or more seismic arrays 215(1-2) over the marine seismic surveyarea 200. It will also be appreciated that, while the survey vessels210(1-2) typically operate on the surface of the sea, the marine seismicsurvey area 200 may refer to a portion of the sea bed. Furthermore,although various implementations described herein are with reference tomarine seismic surveying, it should be understood that someimplementations may be applied to surveys undertaken in freshwater,brackish water and even in land seismic surveying.

The seismic arrays 215(1-2) may include a plurality of seismic sources120 and seismic sensors 125, such as hydrophones, geophones, and thelike, which may be coupled to the survey vessel 210(1-2) by cables 105.The seismic sources 120 and seismic sensors 125 may communicate with asignal processing unit 230 deployed on the survey vessels 210(1-2). Inone implementation, the seismic sources 120 and seismic sensors 125 maycommunicate with the signal processing unit 230 via the cables 105. Forexample, the cables 105 may include wires, fiber-optic cables and thelike, that may allow the seismic sources 120 and the seismic sensors 125to exchange signals with the signal processing unit 230.

However, the seismic sensors 125 and seismic sources 120 may not alwaysbe deployed as a part of the seismic arrays 215(1-2). In someimplementations, the seismic sensors 125 may be deployed on the oceanbottom instead of being towed behind the survey vessels 210(1-2). Forexample, the seismic sensors 125 may be deployed on one or moreocean-bottom cables (“OBC”). The OBCs may be deployed on the seafloor sothat the seismic sensors 125 may record and relay data to the signalprocessing units 230 on the seismic survey vessels 210(1-2).Alternatively, the seismic sources 120 may be deployed on buoys (notshown). In another alternative implementation, the seismic sources 120may be towed by a second vessel (not shown).

The survey vessels 210(1-2) may tow the seismic arrays 215(1-2) along aplurality of sail lines, such as the two sail lines 220(1-2). Althoughonly two sail lines 220(1-2) are shown in FIG. 2, it should beunderstood that surveying the marine seismic survey area 200 may requiremore than two sail lines 220(1-2). For example, a survey covering anarea of 40×70 miles may require about 160 sail lines 220(1-2), with eachsail line 220(1-2) capturing about 1300 feet of subsurface coverageperpendicular to the direction of boat travel. Furthermore, although thetwo sail lines 220(1-2) shown in FIG. 2 are depicted as beingapproximately adjacent, it should be understood that the two sail lines220(1-2) may not be adjacent. In fact, the two sail lines 220(1-2) mayeven overlap.

FIGS. 3A and 3B illustrate a side view of the survey vessel 210(1) and aportion of the seismic array 215(1) at two different locations. Inoperation, the seismic source 125 shown in FIG. 3A may provide anacoustic signal 300(1) that propagates to a seismic sensor 310(1)through a reflection point 320 located on a sea floor 325 between theseismic source 125 and the seismic sensor 310(1). Similarly, in FIG. 3B,the seismic source 125 may provide an acoustic signal 300(2) thatpropagates to a seismic sensor 310(2) through a reflection point 330. Asillustrated in FIGS. 3A and 3B, the sea floor 325 appears to be flat andso the reflection points 320, 330 are half-way between the seismicsource 125 and the seismic sensors 3110(1-2). However, it should beunderstood that the reflection points 320, 330 may not necessarily belocated half-way between the seismic source 125 and the seismic sensors310(1-2). For example, a dipping sea floor 325 may change the locationof the reflection points 320, 330.

In one implementation, signals may be generated by the seismic sensors310(1-2) in response to receiving the reflected and/or refractedacoustic signals 300(1-2). The generated signals may then be transmittedto the signal processing unit 230 on the survey vessel 210(1-2). Thesignal processing unit 230 may in turn use the transmitted signals toform one or more traces representative of the reflected and/or refractedacoustic signals 300(1-2). The signals and/or the traces may be storedon any storage medium, including, but not limited to, recording tape,magnetic disks, compact disks, and DVDs. In some implementations, thesignals may be transmitted to an on-shore facility in addition to or inlieu of storing the signals and/or the traces.

The traces may be grouped according to the location of the reflectionpoints 320, 330. For example, the marine seismic survey area 200 may bedivided into a plurality of midpoint cells 401, as shown in FIG. 4. Thesignals provided by the seismic array 215(1) during a single pass overthe marine seismic survey area 200 may have reflection points 320 (shownin FIG. 3A) that are distributed in a band 405(1) of midpoint cells 401.Similarly, the signals provided by the seismic array 215(2) during asingle pass over the marine seismic survey area 200 may have reflectionpoints 330 (shown in FIG. 3B) that are distributed in a band 405(2) ofmidpoint cells 401. Although the bands 415 (1-2) shown in FIG. 4 aredepicted as adjacent, it should be understood that in someimplementations the bands 415 (1-2) do not necessarily have to beadjacent.

Traces having a common midpoint cell 401 may be combined into a singledata set to form a common midpoint (CMP) gather. This process maycommonly be referred to as bringing the traces to a common midpoint. Themidpoint cells 401 may be combined into cross-line groups 415(1-2). Inone implementation, traces corresponding to the midpoint cells 401 inthe cross-line group 415(1) may be combined to form a firstsail-line/cross-line gather. Similarly, traces corresponding to themidpoint cells 401 in the cross-line group 415(2) may be combined toform a second sail-line/cross-line gather. In another implementation,traces associated with different sail lines 220(1-2) may be combined.For example, the traces corresponding to the midpoint cells 401 in thecross-line groups 415 (1-2) may all be combined to form a cross-linegather.

FIG. 5 illustrates a plurality of signal paths 500(1-3) from a surface505 through a common midpoint cell 510 on a sea floor 520 and back tothe surface 505. Each signal path 500(1-3) has a corresponding offset530(1-3), which represents a horizontal separation of the seismic sourceand seismic sensor. Thus, for a water layer 540 having a depth Z and avelocity of sound in water, or V_(w), an acoustic signal that propagatesalong the signal paths 500(1-3) has a corresponding travel time T₁₋₃given by the formula T₁₋₃=(T₀ ²+X₁₋₃ ²/V_(w) ²)^(1/2), where X₁₋₃ is thelength of the corresponding offset 530(1-3) and T₀=2Z_(w)/V_(w) is thevertical two-way travel time, i.e., the travel time of an acousticsignal propagating along the line 550. The vertical two-way travel timemay also be referred to as the zero offset travel time.

When combining traces formed from signals that propagate along thesignal paths 500(1-3), a so-called normal move-out (NMO) correction maybe applied to the traces. The NMO correction may include transforming atime coordinate of the traces using the equation T_(0, 1-3)=(T₁₋₃ ²−X₁₋₃²/V_(w) ²)². If the water velocity V_(w) is the same for all the traces,then the NMO-corrected travel times T_(0, 1-3) are all equal to a zerooffset travel time T₀. Grouping and/or combining the NMO-correctedtraces may often improve the representation of the marine seismic surveyarea 200 by, e.g., increasing the signal-to-noise ratio of the data.However, as discussed above, variations in the water conditions, e.g.,water temperature, salinity, and the like, between the plurality of saillines 200(1-2) may cause sail-line-to-sail-line variations in the watervelocity, such that the NMO-corrected travel times T_(0, 1-3) may bedifferent for different traces.

FIGS. 6A, 6B, and 6C illustrate a flow diagram of a method 600 forcorrecting seismic data due to errors caused by variations in the waterconditions in accordance with implementations of various techniquesdescribed herein. It should be understood that while the operationalflow diagram 600 indicates a particular order of execution of theoperations, in some implementations, certain portions of the operationsmight be executed in a different order. Further, in someimplementations, certain steps may be combined into fewer steps or evensimplified into a single step. For example, steps 605-615 may becombined into a single step achieving the same result.

At step 605, a dip correction may be applied to a plurality of observedwater bottom reflection times using an initial reference water velocity.An observed water bottom reflection time may be defined as the smallestobserved two-way traveltime of a seismic event recorded at a seismicsensor and having traveled from the source to the water bottom and tothe sensor. In one implementation, the dip correction may includedetermining the difference between the observed water bottom reflectiontimes and the water bottom reflection times that would have beenobserved at the water bottom having no geologic dip and the samedistance from the source-detector midpoint. This difference may then besubtracted from the observed water bottom reflection times. At step 605,the water bottom may be assumed to have a first estimate of geologicdip. The initial reference water velocity may be an estimate of theaverage water velocity for the acquisition area. In one implementation,the initial reference water velocity may be a function of CMP.

At step 610, an NMO correction may be applied to the dip correctedobserved water bottom reflection times using the initial reference watervelocity.

At step 615, a CMP bin centering correction may be applied to the NMOcorrected, dip corrected observed water bottom reflection times. Thisstep may be configured to simulate a common point for the dippingreflector for all recorded water bottom reflection times whosesource-detector midpoints are within the bin. In one implementation, theCMP bin centering correction may include calculating the differencebetween zero offset water bottom reflection times with source anddetector at the recorded source-detector midpoint and zero offset waterbottom reflection times at the CMP bin midpoint using the first estimateof geologic dip at each CMP and the initial reference water velocity.This difference may then be subtracted from the NMO-corrected,dip-corrected observed water bottom reflection times. For purposes ofthe CMP bin centering correction, the reflector may be assumed to beplanar for the water bottom reflection times with source-detectormidpoints within the CMP bin. As a result, the output of step 615 may beCMP bin-centered, NMO-corrected, dip-corrected observed water bottomreflection times.

At step 620, an estimate of zero offset water bottom reflection timesmay be determined using the CMP bin centered, NMO corrected, dipcorrected observed water bottom reflection times. A zero offset waterbottom reflection time may be defined as an estimate of the water bottomreflection time of a seismic event that would have been recorded if thesource and detector had been coincident at a specified x-y coordinate,such as the midpoint between the actual source and detector locations,or the center of a CMP bin. A zero offset water bottom reflection timemay be associated with a certain water velocity because reflection timedepends on the velocity. As such, the estimate of zero offset waterbottom reflection times may be determined by determining a curve that isa best fit of the CMP bin centered corrected, NMO corrected, dipcorrected observed water bottom reflection times and residual moveout.The curve may be best fitted using a least squares equation. Forexample, a residual moveout curve and zero offset water bottomreflection times may be best fitted into the CMP bin centered, NMOcorrected, dip corrected water observed water bottom reflection times.In one implementation, a function ε of the desired curves and waterbottom reflection times may be formulated as follows:ε=|τ_(ij) −t _(j)+α_(j) X _(ij) ²|²where τ_(ij) is the i^(th) CMP bin centered, NMO corrected, dipcorrected observed water bottom reflection time of the CMP bin centered,NMO corrected, dip corrected water observed water bottom reflectiontimes in the j^(th) CMP, t_(j) is the zero offset water bottomreflection times estimate of the j^(th) CMP, and α_(j) is the residualmoveout parameter of the j^(th) CMP such that α_(j) is smooth. Thefunction ε may then be minimized according to various numerical analysismethods, such as those described in Press, W., et al., 1996, NumericalRecipes in Fortran, 2^(nd) ed, Cambridge University Press.

The residual moveout may refer to a small amount of NMO that remainsafter the NMO correction applied at step 610. The NMO correction appliedat step 610 may be incomplete because the velocity used for the NMOcorrection may be incorrect. As such, the residual moveout and zerooffset water bottom reflection times may be determined using the CMPbin-centered, NMO-corrected, dip-corrected observed water bottomreflection times as described above.

At step 625, a CMP may be selected. At step 630, a plane that best fitsover the estimate of zero offset water bottom reflection timesmultiplied by one-half the initial reference water velocity thatcorrespond to the selected CMP and a predetermined set of CMP'ssurrounding the selected CMP may be determined. At step 635, theorientation of the plane may be considered as a second estimate ofgeologic dip at the selected CMP. In one implementation, the angle whosesine is the tangent of the slope of the plane in its steepest directionmay be considered as a second estimate of geologic dip at the selectedCMP. This process may also be referred to as migration. FIG. 8Aillustrates a cross sectional view of a plurality of signal pathsperpendicular to the direction of geologic strike. More specifically,d₁, d₂, d₃ indicate the various distances from the water surface 810 tothe plane 820 of the water bottom surface referred to at step 635. Thatis, d₁, d₂, d₃ represent the zero offset times multiplied by ½ thereference water velocity. α represents the angle of the slope of theplane in the direction of the plane's maximum slope. FIG. 8B illustratesa cross sectional view of a plurality of signal paths perpendicular tothe direction of geologic strike. In this figure, the water surface 810and d₁, d₂, d₃ are the same as shown in FIG. 8A. φ represents the anglewhose sine is the tangent of the slope of the plane 820 in its steepestdirection.

Using step 638, steps 625-635 may then be repeated for all CMP's togenerate a second estimate of geologic dip for each CMP. The geologicdip may provide a value of the direction and slope of the water bottomsurface. As such, each CMP may have its own estimate of geologic dip.

At step 640, the observed water bottom reflection times may be formedinto groups. In one implementation, the observed water bottom reflectiontimes may be grouped according to the method with reference to FIG. 4.In another implementation, the groups may be formed such that a subsetof each group may consist of a single-cable sail line cross line group.In yet another implementation, the grouping may be based on a sail linethat have source-detector midpoints disposed in a line of cells alongthe perpendicular axis of a grid that has its axes approximately alongand perpendicular to the track of the sail line. The grid used toconstruct this grouping may be independent of the CMP grid used inmethod 600. In general, if the CMP grid and the grouping grid would haveroughly the same orientation, then it may not be necessary to have aseparate grid for the grouping. However, when sail lines are not roughlyparallel, e.g., when a survey is acquired with two or more non-parallelorientations of the sail lines, it may be necessary to have differentgrids for the forming of groups. The point of the grouping is that eachgroup may have a distribution of offsets over the nominal range ofacquired offsets and that the source-detector midpoints of a group be inproximity to each other.

At step 645, a dip correction may be applied to the observed waterbottom reflection times using a model water velocity and the secondestimate of geologic dip computed for each CMP at step 635. The modelwater velocity may be a constant or a function of CMP. The model watervelocity may be specified by the user or calculated in a separateprocedure. In one implementation, the model water velocity may be thewater velocity that the user believes is the best NMO velocity to whichthe observed water bottom reflection times will be “layer replaced,”which is a process that will be described in more detail in theparagraphs below.

At step 650, an NMO correction may be applied to the dip correctedobserved water bottom reflection times using the model water velocity.

At step 655, a CMP bin centering correction may be applied to the NMOcorrected, dip corrected observed water bottom reflection times usingthe geologic dip estimates computed for each CMP computed at step 635and the model water velocity.

At step 660, values for t_(i) and Δs_(i) that minimize e as expressed inthe following equation may be solved.e=Σ|t _(nobs,i)−(t _(i) +Δs _(i) a(t _(m) ,s _(m)))|^(r)

-   -   1≦r≦2    -   where a(t_(m),s_(m))=t_(mx) ²/(s_(m)t_(m)),    -   and t_(mx) ²=t_(m) ²+(x_(nobs)s_(m))²        where r is the “norm” of the solution. When r is 1, i.e., an L−1        norm, e, which is a measure of goodness of fit, is a solution of        the least absolute deviation. When r is 2, it is a least squares        solution. t_(nobs,i) is a CMP-bin centered, NMO corrected, dip        corrected observed water bottom reflection time of the i^(th)        iteration, i.e., the i^(th) iteration from steps 640 through        696. t_(i) is a function of CMP bin and is an estimate for the        i^(th) iteration of the zero offset water bottom reflection time        that would have been observed with source and detector        coincident at the CMP midpoint and the model water velocity; and        Δs_(i) is a function of group, i.e., a group of water bottom        reflection times may be associated with a single value of        Δs_(i). In other words, Δs_(i) is the estimate of the difference        in slowness between the observed water bottom reflection times        and the water bottom reflection times that would have been        observed had the water velocity been the model water velocity,        i.e., for the i^(th) iteration. t_(m) is the model time, which        is the estimate of zero offset water bottom reflection time at        the midpoint of the CMP bin when the water velocity is equal to        the model water velocity. The model time t_(m) for the first        iteration is the t_(j)'s calculated at step 620. s_(m) is the        model slowness, which may be defined as the reciprocal of the        model water velocity. a is defined as a non-linear function of        the model zero offset water bottom reflection time t_(m) and the        model slowness s_(m), which themselves are functions of CMP bin.        t_(mx) is defined as t_(m) with inverse moveout at the model        slowness, s_(m), and the distance between the source and        detector of the time t_(nobs,i), which is x_(nobs).

At step 665, an interim observed slowness s_(obs,i) may be solved basedon the following equation s_(obs,i)=s_(a)+Δs_(i), where s_(a) is theaverage of s_(m) taken for the observed water bottom reflection times ofthe group. The interim observed slowness s_(obs,i) is the reciprocal ofobserved water velocity, which is an estimate of the average watervelocity experienced by the seismic energy traveling through the waterduring acquisition, i.e., the water velocity “observed” in the acquiredseismic data.

At step 670, a CMP may be selected. At step 675, a plane that best fitsover the zero offset water bottom reflection times t_(i) multiplied byone-half the model water velocity corresponding to the selected CMP anda predetermined set of CMP's surrounding the selected CMP may bedetermined. At step 680, the orientation of the plane may be consideredas a third estimate of geologic dip at the selected CMP. In oneimplementation, the angle φ whose sine is the tangent of the slope ofthe plane 820 in its steepest direction may be considered as a thirdestimate of geologic dip at the selected CMP. The angle φ and the plane820 are described in more detail in the above paragraphs with referenceto FIGS. 8A and 8B. Using step 682, steps 670-680 may then be repeatedfor all CMP's to generate a third estimate of geologic dip for each CMP.

At step 685, a dip correction may be applied to the observed waterbottom reflection times using the third estimates of geologic dip andthe interim observed water velocity, which is the reciprocal of theslowness computed at step 665.

At step 690, an NMO correction may be applied to the dip correctedobserved water bottom reflection times using the interim observed watervelocity.

At step 692, the NMO corrected, dip corrected observed water bottomreflection times may be scaled according to the change from the interimobserved slowness s_(obs,i) and the model slowness s_(m) using thefollowing relationship.t _(b) |s _(obs,i) =t _(r) |s _(m)where t_(b) is the NMO corrected, dip corrected observed water bottomreflection times, and t_(r) is the scaled result time.

At step 694, an inverse NMO correction may be applied to the scaled, NMOcorrected, dip corrected observed water bottom reflection times usingthe model slowness s_(m).

At step 696, an inverse dip correction may be applied to the inverse NMOcorrected, scaled, NMO corrected, dip corrected observed water bottomreflection times using the third estimates of geologic dip and the modelslowness s_(m). The output of this step is the water bottom reflectiontimes that would have been observed if the water velocity (slowness)were s_(m) and the actual observed velocity were the interim observedwater velocity, which is the reciprocal of the interim observed slownesss_(obs,i) computed at step 665. In this manner, the observed waterbottom reflection times may be layer replaced.

At step 698, steps 640-696 may be repeated using the layer replacedwater bottom reflection times in place of the observed water bottomreflection times and substituting s_(obs,i)=s_(obs,i-1)+Δs_(i) fors_(obs,i)=s_(a)+Δs_(i), where s_(obs,i-1) is s_(obs,i) of the previousstep 665. This repetition may be performed until the changes in theinterim observed slowness s_(obs,i) from subsequent iterations are aboutzero. As such, the final output of method 600 may be a final value ofobserved water velocity, which is the reciprocal of the final interimobserved slowness s_(obs,i). As mentioned above, the observed watervelocity may be defined as the localized average velocity of the waterat the time of acquisition. The observed water velocity may also bedescribed as the velocity that, when replaced with the model watervelocity in a layer replacement calculation, changes the observed waterbottom reflection times to those that would have been observed had thewater velocity at the time of acquisition been the model water velocity.

In one implementation, at step 645, the dip correction may be applied tothe layer replaced water bottom reflection times using estimates ofgeologic dip determined at step 670 in the second and subsequentiterations. In another implementation, at step 650, the NMO correctionmay be applied to the dip corrected, layer replaced water bottomreflection times in the second and subsequent iterations. In yet anotherimplementation, at step 655, the CMP bin centering correction may beapplied to the NMO corrected, dip corrected, layer replaced water bottomreflection times in the second and subsequent iterations. In stillanother implementation, for second and subsequent iterations, t_(m) isthe t_(i) computed in step 660 of the previous iteration.

In one implementation, the model water velocity may be the NMO velocityequivalent to the RMS water velocity, or even the RMS water velocityitself, which may be defined as the root mean square water velocity ofall the layers. The RMS water velocity may be calculated from thethickness and velocity of all the layers as expressed in the followingequation:V _(rms)=[Σ_(i) V _(i) ² t _(i)|Σ_(i) t _(i)]^(1/2),where z_(i) is the layer thickness, V_(i) is velocity andt_(i)=z_(i)/V_(i) is the vertical travel time across the layer ofthickness z_(i) and velocity V_(i).

In another implementation, the model water velocity may be constant forthe first iteration. An RMS water velocity may then calculated for eachCMP from the model zero offset water bottom reflection times t_(m) and aspecified layering of velocities in the water layer. This RMS watervelocity (as a function of CMP) may then be used as the model watervelocity in subsequent iterations.

As such, although various implementations described herein may be usedto estimate the observed RMS water velocity associated with anyreflecting horizon, one primary use is in finding the velocity of soundin the water layer as it varies laterally and temporally during therecording of marine 3-D surveys.

In another implementation, the observed water velocity from the firstiteration where the model water velocity is a constant may be spatiallysmoothed to produce another model water velocity that is close to theobserved water velocity. The spatially smoothed observed water velocitymay then be used as the model water velocity in subsequent iterations.Because the difference between the observed velocity and the spatiallysmoothed observed water velocity is small, layer replacement of theobserved water bottom reflection times using the observed velocity andthe spatially smoothed observed water velocity may not move the seismicdata very much, which may be desirable.

FIG. 7 illustrates a computing system 700, into which implementations ofvarious technologies described herein may be implemented. The computingsystem 700 may include one or more system computers 730, which may beimplemented as any conventional personal computer or server. However,those skilled in the art will appreciate that implementations of varioustechnologies described herein may be practiced in other computer systemconfigurations, including hypertext transfer protocol (HTTP) servers,hand-held devices, multiprocessor systems, microprocessor-based orprogrammable consumer electronics, network PCs, minicomputers, mainframecomputers, and the like.

The system computer 730 may be in communication with disk storagedevices 729, 731, and 733, which may be external hard disk storagedevices. It is contemplated that disk storage devices 729, 731, and 733are conventional hard disk drives, and as such, will be implemented byway of a local area network or by remote access. Of course, while diskstorage devices 729, 731, and 733 are illustrated as separate devices, asingle disk storage device may be used to store any and all of theprogram instructions, measurement data, and results as desired.

In one implementation, seismic data from the receivers may be stored indisk storage device 731. The system computer 730 may retrieve theappropriate data from the disk storage device 731 to process seismicdata according to program instructions that correspond toimplementations of various techniques described herein. The programinstructions may be written in a computer programming language, such asC++, Java and the like. The program instructions may be stored in acomputer-readable medium, such as program disk storage device 733. Suchcomputer-readable media may include computer storage media andcommunication media. Computer storage media may include volatile andnon-volatile, and removable and non-removable media implemented in anymethod or technology for storage of information, such ascomputer-readable instructions, data structures, program modules orother data. Computer storage media may further include RAM, ROM,erasable programmable read-only memory (EPROM), electrically erasableprogrammable read-only memory (EEPROM), flash memory or other solidstate memory technology, CD-ROM, digital versatile disks (DVD), or otheroptical storage, magnetic cassettes, magnetic tape, magnetic diskstorage or other magnetic storage devices, or any other medium which canbe used to store the desired information and which can be accessed bythe system computer 730. Communication media may embody computerreadable instructions, data structures, program modules or other data ina modulated data signal, such as a carrier wave or other transportmechanism and may include any information delivery media. The term“modulated data signal” may mean a signal that has one or more of itscharacteristics set or changed in such a manner as to encode informationin the signal. By way of example, and not limitation, communicationmedia may include wired media such as a wired network or direct-wiredconnection, and wireless media such as acoustic, RF, infrared and otherwireless media. Combinations of any of the above may also be includedwithin the scope of computer readable media.

In one implementation, the system computer 730 may present outputprimarily onto graphics display 727, or alternatively via printer 728.The system computer 730 may store the results of the methods describedabove on disk storage 729, for later use and further analysis. Thekeyboard 726 and the pointing device (e.g., a mouse, trackball, or thelike) 725 may be provided with the system computer 730 to enableinteractive operation.

The system computer 730 may be located at a data center remote from thesurvey region. The system computer 730 may be in communication with thereceivers (either directly or via a recording unit, not shown), toreceive signals indicative of the reflected seismic energy. Thesesignals, after conventional formatting and other initial processing, maybe stored by the system computer 730 as digital data in the disk storage731 for subsequent retrieval and processing in the manner describedabove. While FIG. 7 illustrates the disk storage 731 as directlyconnected to the system computer 730, it is also contemplated that thedisk storage device 731 may be accessible through a local area networkor by remote access. Furthermore, while disk storage devices 729, 731are illustrated as separate devices for storing input seismic data andanalysis results, the disk storage devices 729, 731 may be implementedwithin a single disk drive (either together with or separately fromprogram disk storage device 733), or in any other conventional manner aswill be fully understood by one of skill in the art having reference tothis specification.

While the foregoing is directed to implementations of varioustechnologies described herein, other and further implementations may bedevised without departing from the basic scope thereof, which may bedetermined by the claims that follow. Although the subject matter hasbeen described in language specific to structural features and/ormethodological acts, it is to be understood that the subject matterdefined in the appended claims is not necessarily limited to thespecific features or acts described above. Rather, the specific featuresand acts described above are disclosed as example forms of implementingthe claims.

1. A method for processing seismic data to correct for errors caused byvariation in water conditions, comprising: (a) applying a dip correctionto a plurality of observed water bottom reflection times using a modelwater velocity and an estimate of geologic dip; (b) applying a normalmoveout (NMO) correction to the dip corrected observed water bottomreflection times using the model water velocity; (c) applying a commonmid point (“CMP”) bin centering correction to the NMO corrected, dipcorrected observed water bottom reflection times using the model watervelocity and the estimate of geologic dip; (d) solving for Δs_(i), whichis an estimate of the difference in slowness between the observed waterbottom reflection times and the water bottom reflection times that wouldhave been observed had the water velocity been the same as the modelwater velocity; (e) solving for an estimate of observed water velocitybased on s_(obs,i)=s_(m)+Δs_(i), where s_(obs,i) is an estimate ofobserved slowness and s_(m) is defined as the model slowness; (f) layerreplacing the observed water bottom reflection times using the estimateof the observed water velocity and the model water velocity; and (g)repeating steps (a) to (f) using the layer replaced observed waterbottom reflection times until the changes in the estimate of observedwater velocity approach zero.
 2. The method of claim 1, furthercomprising correcting the seismic data using the estimate of theobserved water velocity.
 3. The method of claim 1, wherein the modelslowness is the reciprocal of the model water velocity.
 4. The method ofclaim 1, further comprising forming the observed water bottom reflectiontimes into groups such that a subset of each group comprises a singlecable sail line cross line group.
 5. The method of claim 1, furthercomprising forming the observed water bottom reflection times intogroups such that each group has a distribution of offsets over a rangeof acquired offsets and that the source-detector midpoints of each groupbe in proximity to each other.
 6. The method of claim 1, wherein themodel water velocity is a predetermined value.
 7. The method of claim 1,wherein Δs_(i) is solved by solving for t_(i) and Δs_(i) that minimize eas expressed in the following equation:e=Σ|t _(nobs,i)−(t _(i) +Δs _(i) a(t _(m) ,s _(m)))|^(r) 1≦r≦2 wherea(t_(m),s_(m))=t_(mx) ²/(s_(m)t_(m)), and t_(mx) ²=t_(m)²+(x_(nobs)s_(m))² e is a measure of goodness of fit, t_(i) is anestimate for the i^(th) iteration of a zero offset water bottomreflection time that would have been observed with a source and adetector coincident at the CMP midpoint and the model water velocity,t_(nobs,i) is a CMP-bin centered, NMO corrected, dip corrected observedwater bottom reflection time of the i^(th) iteration, t_(m) is anestimate of a zero offset water bottom reflection time at the midpointof the CMP bin when the water velocity is equal to the model watervelocity, a is defined as a non-linear function of t_(m) and s_(m), andt _(mx) is defined as t_(m) with inverse moveout at the model slowness,s_(m), and the distance between the source and detector of the timet_(nobs,i), which is x_(nobs).
 8. The method of claim 1, wherein theobserved water velocity is the average water velocity experienced byseismic energy traveling through the water during acquisition.
 9. Themethod of claim 1, further comprising: selecting a CMP; determining aplane that best fits over the model zero offset water bottom reflectiontimes t_(m) multiplied by one half the model water velocitycorresponding to the selected CMP and a predetermined set of CMP'ssurrounding the selected CMP; and considering the angle whose sine isthe tangent of the slope of the plane in its steepest direction as afurther estimate of geologic dip for the selected CMP.
 10. The method ofclaim 9, wherein layer replacing the observed water bottom reflectiontimes comprises: applying a dip correction to the observed water bottomreflection times using the further estimate of geologic dip of each CMPand the estimate of observed water velocity; applying an NMO correctionto the dip corrected observed water bottom reflection times using theestimate of observed water velocity; scaling the NMO corrected, dipcorrected observed water bottom reflection times according tot_(b)/s_(obs,i)=t_(r)/s_(m), where t_(b) is the NMO corrected, dipcorrected observed water bottom reflection times and t_(r) is the scaledresult time; applying an inverse NMO correction to the scaled, NMOcorrected, dip corrected observed water bottom reflection times usingthe model slowness s_(m); applying an inverse dip correction to theinverse NMO corrected, scaled, NMO corrected, dip corrected observedwater bottom reflection times using the further estimate of geologic dipfor each CMP and the model slowness s_(m).
 11. The method of claim 1,wherein the estimate of geologic dip is computed using dip correction.12. The method of claim 1, wherein the estimate of geologic dip iscomputed using dip correction and CMP bin centering correction.
 13. Acomputer-readable medium having stored thereon computer-executableinstructions which, when executed by a computer, cause the computer to:(a) apply a dip correction to a plurality of observed water bottomreflection times using a model water velocity and an estimate ofgeologic dip; (b) apply a normal moveout (NMO) correction to the dipcorrected observed water bottom reflection times using the model watervelocity; (c) apply a common mid point (“CMP”) bin centering correctionto the NMO corrected, dip corrected observed water bottom reflectiontimes using the model water velocity and the estimate of geologic dip;(d) solve for Δs_(i), which is an estimate of the difference in slownessbetween the observed water bottom reflection times and the water bottomreflection times that would have been observed had the water velocitybeen the same as the model water velocity; (e) solve for an estimate ofobserved water velocity based on s_(obs,i)=s_(m)+Δs_(i), where s_(obs,i)is an estimate of observed slowness and s_(m) is defined as the modelslowness; (f) layer replace the observed water bottom reflection timesusing the estimate of the observed water velocity and the model watervelocity; and (g) repeat steps (a) to (f) using the layer replacedobserved water bottom reflection times until the changes in the estimateof observed water velocity approach zero.
 14. The computer-readablemedium of claim 13, further comprising computer-executable instructionswhich, when executed by a computer, cause the computer to correct theseismic data using the estimate of the observed water velocity.
 15. Thecomputer-readable medium of claim 13, wherein the observed watervelocity is the average water velocity experienced by seismic energytraveling through the water during acquisition.
 16. Thecomputer-readable medium of claim 13, further comprisingcomputer-executable instructions which, when executed by a computer,cause the computer to select a CMP; determine a plane that best fitsover the model zero offset water bottom reflection times t_(m)multiplied by one-half the model water velocity corresponding to theselected CMP and a predetermined set of CMP's surrounding the selectedCMP; and consider the angle whose sine is the tangent of the slope ofthe plane in its steepest direction as a further estimate of geologicdip for the selected CMP.
 17. The computer-readable medium of claim 13,wherein the estimate of geologic dip is computed using dip correction.18. A computer system, comprising: a processor; and a memory comprisingprogram instructions executable by the processor to: (a) apply a dipcorrection to a plurality of observed water bottom reflection timesusing a model water velocity and an estimate of geologic dip; (b) applya normal moveout (NMO) correction to the dip corrected observed waterbottom reflection times using the model water velocity; (c) apply acommon mid point (“CMP”) bin centering correction to the NMO corrected,dip corrected observed water bottom reflection times using the modelwater velocity and the estimate of geologic dip; (d) solve for Δs_(i),which is an estimate of the difference in slowness between the observedwater bottom reflection times and the water bottom reflection times thatwould have been observed had the water velocity been the same as themodel water velocity; (e) solve for an estimate of observed watervelocity based on s_(obs,i)=s_(m)+Δs_(i), where s_(obs,i) is an estimateof observed slowness and s_(m) is defined as the model slowness; (f)layer replace the observed water bottom reflection times using theestimate of the observed water velocity and the model water velocity;and (g) repeat steps (a) to (f) using the layer replaced observed waterbottom reflection times until the changes in the estimate of observedwater velocity approach zero.
 19. The computer system of claim 18,wherein the estimate of geologic dip is computed using dip correction.20. The computer system of claim 18, wherein the observed water velocityis the average water velocity experienced by seismic energy travelingthrough the water during acquisition.